Edges with mean crease set to 1 are still rounded by a subsurf modifier
Make sure the inner edges are creased at the corners as well. For example:
With only outer edges creased

With inner edges creased

As a bonus, you might want to drop an Edge Split modifier on as well to get your object smooth and remove the stretched shades that creasing and subsurf will produce. Also see How to smooth shade an object while retaining hard edges?.